Field of the Invention
The present invention relates to image sensing devices, and more particularly to an image sensing device having reduced reflective index (n) variations between the color filters and the microlenses therein.
Description of the Related Art
Image sensing devices are necessary components in many optoelectronic devices, including digital cameras, cellular phones, and toys. Conventional sensing devices include both charge coupled device (CCD) image sensors and complementary metal oxide semiconductor (CMOS) image sensors.
An image sensor typically includes a plane array of pixel cells, wherein each pixel cell comprises a photogate, photoconductor or a photodiode having a doped region for accumulating a photo-generated charge. A periodic pattern of dyes or pigment of different colors such as red (R), green (G), or blue (B) is superimposed over the plane array of pixel cells. This pattern is known as a color filter array (CFA). A plurality of microlenses of a square or a circular shape can be optionally superimposed over the color filter array (CFA) to focus light onto one initial charge accumulation region of each of the pixel cells. Using the microlenses may significantly improve the photosensitivity of the image sensor.
However, due to the refractive index (n) variations between materials of the color filters of different colors such as R, G, or B, and materials of the microlenses formed thereover, undesired refraction can happen when incident light arrives at an interface between the microlenses and the color filters. Therefore, energy of the incident light passing through the microlenses and color filters is reduced and sensitivity of the pixel cells of the image sensing device is thus reduced.